A carbon (base) body coated with pyrolytic boron nitride is known, and is generally classified, depending on the manufacture method, into molded, extruded and CIP (cold isostatic press). Graphite, which constitutes this carbon body, has a structure in which layers, each consisting of honeycomb-patterned plane with six-member carbon rings condensing together, are piled up, and this piling is maintained by very weak intermolecular pull force (namely, Van der Waals force), which works in the direction vertical to the layers. Each carbon atom constituting a member of any one of said six-member carbon rings is bonded with three neighboring carbon atoms thereby forming a highly rigid center of equiangularly oriented three covalent bonds in a plane.
Such special carbonaceous bodies are used to make structural members of a reacting furnace and heater members, and the ones made by CIP are the most commonly used. The CIP carbonaceous body, owing to its manufacturing method, is characteristic in that its six-member rings form a three-dimensionally arranged random body, and is hence isotropic. Also this special carbonaceous body can be made to have particular ranges of thermal expansion coefficient and electric resistivity, by selecting the manner by which a raw material coke is admixed.
The non-CIP carbonaceous bodies, that is, the molded carbonaceous body and the extruded carbonaceous body are built of coarse grains so that minute defects and pores are many in their surfaces and inside; hence they are not preferred to make structural members of a reacting furnace and heater members. With regard to the CIP carbonaceous body, although it is called isotopic graphite because it is manufactured by CIP method, it is in fact not easy to make a perfectly isotopic body owing to negative effects arising from shaping process and the self weight. In particular, the greater the size of the carbon block to be made is, the greater does the consequence of the self weight become, whereby problematic anisotropy is increased.
In recent years, in order to reduce the production cost, improvement in throughput is called for, and demand for large-sized carbon blocks is increasing. The reason for this is that although a large-sized carbon block is anisotropic in thermal expansion coefficient, a smart manner of dissecting it at once into many small-sized blocks can lead to a better cost reduction. Also by cunningly making use of the anisotropic nature of the large-sized carbon block, it becomes possible to make a carbon heater with increased durability.
Now, a coating layer made of pyrolytic boron nitride (hereinafter referred to as “PBN”) is produced by CVD method, and the PBN layer (film) has properties of high insulation, high heat resistance and high flexibility. The structure of PBN is of a hexagonal system, similar to graphite, and the physical properties show an extensive anisotropy between the direction parallel to the growth plane [plane direction (a)] and a direction vertical to the growth plane [thickness-wise direction (c)]. Especially with respect to the thermal expansion coefficient, the difference is by about ten-fold: e.g., about 3.0×10−6 [1/deg.C.] in direction (a) and about 30×10−6 [1/deg.C.] in direction (c).
Now, PBN has a characteristic that the higher the degree of its orientation is, the greater does the contribution of covalent bonds in direction (a) become, whereby the thermal expansion coefficient becomes lower; on the other hand the lower the degree of its orientation is, the greater does the contribution of intermolecular force in dictions c becomes, whereby the heat expansion coefficient becomes higher; so that by changing the degree of orientation (anisotropy) it becomes possible to modify the thermal expansion coefficient
When a carbon body is coated with a PBN layer with such useful properties, it becomes possible to use the carbon body even in a corrosive environment such as H2 or NH3 at high temperatures; hence a carbon body coated with PBN is used as a carbon heater and the like. However, the difference in the thermal expansion coefficient between the carbon body and the PBN layer would cause deformation, peeling off and cracking.
When used as a carbon heater, a PBN-coated carbon body experiences rapid raising and lowering of temperature, as it is desirable to shorten the operation so as to increase the throughput; as a consequence of this rapid operation, a temperature profile occurs in the heater to give rise to a buildup of thermal stress, which causes a crack in the PBN layer to expose the inner carbon body to be consumed.
In order to prevent the deformation, peeling off, cracking, etc., there have been proposed various countermeasures. For example, IP Publication 1 discloses a method for suppressing peeling off and cracking of the PBN layer through weakening of stress concentration, which is achieved by decreasing the surface roughness of the carbon body and chamfering the corners to a radius of curvature of 0.5 mm or greater. However, although this method may minimize the stress concentration at the corner areas, it is not possible by the method to mitigate the thermal stress which is created in the areas where the surface is highly flat by the effects of the thermal expansion coefficient difference and the temperature difference.
Also, IP Publication 2 discloses a method for forming a crack-free coating layer on a carbon base body by laying a PBN layer having a relatively low thermal expansion coefficient which is nearly equal to that of the carbon base body. However, owing to the fact that the thermal expansion coefficient of the PBN layer is necessarily greater than that of the carbon base body, this method cannot but fail, for when thermal stress is built by the repetition of temperature rising and falling, the stress causes a tensile stress to work on the PBN layer with a consequence that the latter is cracked.
As such, although the methods taught by the prior publications are effective in reducing the thermal stress between the PBN layer and the carbon base body, the effect obtained is not yet sufficient to solve the problem, for when the temperature rising and falling is repeated whereby the thermal load is imposed, the PBN layer is cracked.